I am getting the following error:

TypeError: cannot concatenate object of type '<class 'str'>'; only Series and DataFrame objs are valid

In the following block:

for idx, day_row in curr_df.iterrows():
    av_last_month, rate_end_last_mon = last_months_avs(df_to_date, day_row['From_Currency'], day_row['To_Currency'], day_row['Obs_Date'])

concated_data = ({'Obs_Date': day_row['Obs_Date'], 'From_Currency': day_row['From_Currency'], 
'To_Currency': day_row['To_Currency'], 'Rate': day_row['Rate'],
'PE_Rate': av_last_month, 'Mon_End_Rate': rate_end_last_mon})

data = pd.concat(concated_data, ignore_index=True)

new_df = pd.DataFrame(data)